Dragline excavators have long booms which comprise a number of main tubular chords connected by tubular lacing. The tubular lacing is connected to the main chords at cluster joints. FIG. 1 illustrates a typical cluster joint and the complex intersection between the lacings and the chord. Dragline booms are called upon to support large dynamic loads. Stresses tend to be concentrated at the cluster joint weldments at which the lacing is connected to the main chord. Over time, these stresses cause fatigue failures at the cluster joints. With increased productivity demands and cost of machine down time, failure of cluster joints on the current tubular dragline boom design requires temporary weld repair until a sufficiently long outage is available to lower the boom and complete a repair under controlled conditions. Such temporary weld repair may be performed under adverse conditions. Even under controlled conditions with the boom lowered, the fatigue life of the repaired cluster joint is undesirably short.
Aside from the limited maintenance schedules which generally preclude lowering the boom and the outage cost associated with such an operation, lowering the boom is viewed by operators as a dangerous exercise exposing the operator to a potentially high risk event with significant financial consequences.
Conventional tubular boom structures typically have about 10% of the welds hidden from view by the overlapping nature of the cluster joint design. This makes routine inspection impossible. Even locating cracks by pressurizing chords of the boom and finding air leaks can be difficult.
Numerous failures of cluster joints on tubular booms have occurred throughout the world, some leading to catastrophic collapse of the boom.
Failures of cluster joints may be initiated by the growth of fatigue cracks at welds connecting the secondary lacings and the main chord. These regions are associated with high stress concentrations arising from the cluster geometry as well as the presence of weld beads. Where clusters have been weld repaired in situ, the fatigue life of the joint can be reduced due to incomplete penetration of the weld, inclusion of contaminants, irregular internal and external weld geometry and the generation of high residual stresses due to the welding process. If a failure at a cluster involves the main chord material it can be necessary to cut a window to gain access to the main chord and allow for repair of the chord through the window. After the repair is completed the window must be re-inserted and welded in place. This repair is difficult to conduct and causes damage to the cluster as a consequence of the constraints of the repair i.e. weld profile grinding or post weld dressing techniques are difficult to apply.
There is a need for dragline booms that have increased service lives. There is also a need for methods for repairing failures or defects in dragline booms in situ or when the boom is lowered, which avoid at least some of the disadvantages of current methods.